1. Field of the Invention
The invention relates to an engine starting method, in particular for an internal combustion engine of a motor vehicle having an internal combustion engine and having at least one electric motor as a drive motor. The invention also relates to a motor vehicle for carrying out an engine starting method.
2. Description of the Related Art
Motor vehicles having an internal combustion engine and an electric motor are known in the prior art and are also referred to as hybrid vehicles. If the internal combustion engine and the electric motor are arranged on one shaft, this arrangement is also referred to as a parallel hybrid.
The motor vehicle may be driven either only by the internal combustion engine, only by the electric motor or by the internal combustion engine and the electric motor simultaneously. The internal combustion engine can be connected to the electric motor by means of a clutch. A transmission can be connected to the electric motor downstream in the torque flow and the drivetrain can be connected to the transmission downstream. The clutch between the internal combustion engine and the electric machine is disengaged when the internal combustion engine is deactivated so that there is no power-transmitting connection between the internal combustion engine and the electric motor. In this operating situation, the electric motor alone drives the vehicle.
The clutch must be engaged if the internal combustion engine is to be started so that the internal combustion engine can be turned over up to the starting rotational speed at which the engine can start of its own accord. Here, however, unpleasant juddering is experienced by the vehicle occupants, because torque that was available for the forward propulsion of the vehicle before the starting of the internal combustion engine suddenly is branched away and no longer available for forward propulsion due to the turning-over of the internal combustion engine.
DE 10 2008 040 662 A1 discloses a system where a clutch torque is raised linearly, then held at a plateau as a function of time, and for thereafter is raised linearly again. The system is intended to determine the breakaway torque of the internal combustion engine, because it changes over the course of the operation of the engine.
This approach, however, entails a method step that fundamentally delays the starting of the engine and also is perceived as being rather unpleasant. Also, the friction power input into the clutch is increased so that the method is not recommended for thermal reasons.